Position relative to carbon decides the extraction method

1. Most metals are found in the Earth's crust as metal compounds, often oxides, mixed with rock.
   1. Extracting a metal means separating it from the other elements in its compound, most often from oxygen.
2. A few very unreactive metals, such as gold and platinum, are found uncombined as the metal itself.
3. In the reactivity series, carbon sits between aluminium and zinc.
   1. Metals less reactive than carbon, such as zinc, iron, tin, lead and copper, can be extracted by heating with carbon.
   2. Metals more reactive than carbon, such as potassium, sodium, calcium, magnesium and aluminium, must be extracted by electrolysis.

Metals below carbon are extracted by heating with carbon

1. When a metal oxide is heated strongly with carbon, the carbon removes the oxygen and leaves the metal: $$metal\; oxide+carbon\to metal+carbon\; dioxide$$
   1. The metal oxide loses oxygen, so it is reduced, while the carbon gains oxygen, so it is oxidised.
2. In a blast furnace, iron ore, coke and limestone are fed in at the top and hot air is blown in near the bottom.
   1. Coke is almost pure carbon, and it burns to heat the furnace and form carbon monoxide: $$2C+O_2\to 2CO$$
3. Carbon monoxide reduces the iron(III) oxide in the ore to molten iron: $${Fe}_2{O}_3+3CO\to 2Fe+3{CO}_2$$
4. Limestone removes sandy impurities by forming slag, which floats on the molten iron and is run off separately.

Copper, a non-ferrous metal, is extracted using carbon

1. Copper is below carbon in the reactivity series, so its compounds can be reduced by carbon.
2. The ore malachite contains copper carbonate, which breaks down on heating to give copper oxide: $${CuCO}_3\to CuO+{CO}_2$$
   1. The copper oxide is then heated with carbon to give copper: $$2CuO+C\to 2Cu+{CO}_2$$
3. Copper made by heating its ore is impure, so it is purified by electrolysis before it is used for electrical wiring.
4. Zinc and lead are other non-ferrous metals below carbon, and zinc can be extracted from zinc oxide in the same way: $$ZnO+C\to Zn+CO$$

Electrolysis extracts metals above carbon but needs more energy

1. Carbon cannot reduce the oxides of metals above carbon, because carbon is less reactive than these metals and cannot take the oxygen from them.
2. These metals are extracted by passing an electric current through a molten compound of the metal.
   1. The compound must be molten so that its ions are free to move to the electrodes.
   2. In an aqueous solution, hydrogen would form at the cathode instead of the reactive metal.
3. Sodium is extracted by electrolysis of molten sodium chloride, which gives sodium at the cathode and chlorine gas at the anode.
4. Electrolysis needs a large amount of electrical energy, so it costs more than reduction with carbon and is used only when carbon cannot do the job.
   1. This is why aluminium smelters are often built near cheap electricity, such as the hydroelectric-powered smelter at Fort William in Scotland.

Aluminium is extracted by electrolysis in molten cryolite

1. The ore bauxite is purified to give aluminium oxide, which is then dissolved in molten cryolite.
   1. Cryolite lowers the operating temperature from over $2000{}^{\circ }C$ to about $950{}^{\circ }C$, which saves a large amount of energy.
2. The cell has carbon electrodes, with the carbon lining of the steel tank acting as the cathode.
3. Positive aluminium ions move to the cathode, where molten aluminium forms and collects at the bottom of the cell.
4. Negative oxide ions move to the anode, where oxygen gas forms.
   1. The oxygen reacts with the hot carbon anodes to form carbon dioxide, so the anodes burn away and must be replaced regularly.
5. The overall change is: $$2{Al}_2{O}_3\to 4Al+3O_2$$